A number of diseases are treated by the transplantation of tissue donated by other humans (allografts) or obtained from animals (xenografts). For example, insulin-dependent diabetes is often treated by transplantation of insulin-secreting pancreatic islet cells. While the transplanted cells may have the capacity to perform the desired function (e.g., secretion of insulin in response to rising levels of glucose), such grafts typically fail as a result of immunological rejection. Shortly after transplantation, cells of the immune system of the recipient recognize the allogeneic or xenogeneic cells as foreign and proceed to attack the graft through both humoral and cellular routes. Allogeneic or xenogeneic cells are initially recognized by the recipient's immune system through antigenic determinants expressed on the surface of the cells. The predominant antigens recognized as “non-self” are major histocompatibility complex class I and class II antigens (MHC class I and class II). MHC class I antigens are expressed on virtually all parenchymal cells (e.g., pancreatic islet cells). In contrast, MHC class II antigens are expressed on a limited number of cell types, primarily B cells, macrophages, dendritic cells, Langerhans cells and thymic epithelium. The interaction of foreign MHC antigens with the T cell receptor on host T cells causes these cells to become activated. Following activation, the T cells proliferate and induce effector functions which result in cell lysis and destruction of the transplanted cells.
For transplantation to be a viable therapeutic option, approaches are needed to inhibit rejection of transplanted cells by the immune system of the recipient. One method for inhibiting this rejection process is by administration of drugs that suppress the function of the immune system. While drugs such as cyclophosphamide and cyclosporin effectively inhibit the actions of the immune system and thus allow graft acceptance, their use can cause generalized, non-specific immunosuppression in the graft recipient which leaves the recipient susceptible to other disorders such as infection and tumor growth. Additionally, administration of immunsuppressive drugs is often accompanied by other serious side effects such as renal failure and hypertension.
It has been shown that it is possible to alter an antigen on the surface of a cell prior to transplantation to “mask” the antigen from normal recognition by cells of the recipient's immune system (see, Faustman and Coe, Science 252:1700-1702 (1991) and WO 92/04033). For example, MHC class I antigens on transplanted cells can be altered by contacting the cells with a molecule which binds to the antigen, such as an antibody or fragment thereof (e.g., a F(ab′)2 fragment) prior to transplantation. This alteration of MHC class I antigens modifies the interaction between the antigens on the cells and T lymphocytes in the recipient following transplantation to thereby inhibit rejection of the transplanted cells. Additional methods for reducing the immunogenicity of an allograft or xenograft to inhibit rejection of the graft following transplantation in a host are needed.
T-cell mediated immune responses are thought to be the primary mechanism of organ transplant rejection and a driving component of various auto-immune diseases. This T-cell mediated immune response is initially triggered by helper T-cells which are capable of recognizing specific antigens. These helper T-cells may be memory cells left over from a previous immune response or naive cells which are released by the thymus and may have any of an extremely wide variety of antigen receptors. When one of these helper T-cells recognizes an antigen present on the surface of an antigen presenting cell (APC) or a macrophage in the form of an antigen-MHC complex, the helper T-cell is stimulated by signals emanating from the antigen-specific T-cell receptor, co-receptors, and IL-1 secreted by the APC or macrophage, to produce IL-2. The helper T-cells then proliferate. Proliferation results in a large population of T-cells which are clonally selected to recognize a particular antigen. T-cell activation may also stimulate B-cell activation and nonspecific macrophage responses. Some of these proliferating cells differentiate into cytotoxic T-cells which destroy cells having the selected antigen. After the antigen is no longer present, the mature clonally selected cells will remain as memory helper and memory cytotoxic T-cells, which will circulate in the body and recognize the antigen should it show up again. If the antigen triggering this response is not a foreign antigen, but a self antigen, the result is auto immune disease; if the antigen is an antigen from a transplanted organ, the result is graft rejection. Consequently, it is desirable to be able to regulate this T cell mediated immune response.
The current paradigms of immunosuppressive agents reflects the progress in understanding the cellular and molecular mechanisms which mediate graft rejection. Six paradigms represent the evolution of immunosuppressive strategies for organ transplantation to date. The proliferation paradigm advances agents which interrupt lymphocyte cell division (azathioprine, cyclophosphamide, mycophenolic acid). The depletion paradigm conscripts drugs that bind to lymphocyte cell surface markers, thereby producing cell lysis and/or inactivation (polyclonal ATGAM and thymoglobulin, and monoclonal OKT3 antilymphocyte antibodies). The cytokine paradigm uses agents that interrupt lymphocyte maturational events; eg, synthesis (calcineurin inhibitors: cyclosporine/tacrolimus), binding to surface receptors (anti-CD25 mAbs), or signal transduction phases of cytokine stimulation (sirolimus). The introduction of calcineurin inhibitors markedly reduces the rate of acute rejection episodes and increases short-term graft survival rates; nephrotoxicity and chronic allograft attrition remain as unanswered challenges. The cyclosporine A (CsA) sparing property of sirolimus permits the use of lower exposure to calcineurin agents, allows for early withdrawal of steroid therapy, and may delay allograft senescence. Furthermore, the combination of SRL with anti-IL-2R mAbs proffers an induction approach which allows prolonged periods of holiday from calcineurin inhibitors. To address the tissue nonselectivity of the calcineurin and mTOR inhibitors, which presumably causes the drug toxicities, new agents are being developed to selectively inhibit the T cell target Janus Kinase 3. In the co-stimulabon paradigm, the accessory signals generated by antigen-presenting cells are interrupted by distinct agents: the receptor conjugate CTLA4-immunoglobulin and anti-B7 or anti-CD40 ligand mAbs. Another set of drugs (selectin blocking agents, anti-ICAM-1 antisense deoxy oligonucleotides, and the lymphocyte homing inhibitor FTY720) seeks to modulate the ischemia-reperfusion injury, which exacerbates cytokine-mediated events in the donor and the subsequent procurement injury and may also accelerate the progression of transplant senescence. Finally, the transplantation tolerance paradigm is based on the development of strategies which distort alloimmune recognition by antigen reactive cells (MHC peptides or proteins), produce anergy (costimulation blockers), functional inactivation, or deletion of antigen-reactive cells (donor bone marrow infusions and gene therapy).
Thus, the common paradigms today focus upon either T-cell expansion or extravasation into the rejected tissue site. However, a relatively ignored component of immune rejection is antigen presentation, which we now document herein as an excellent target for intervention through the use of poorly catabolized polymers.